The latest trend in virii is the use of MBR rootkits to settle themselves in the unused (hidden) sectors of your harddrive, safe from any formatting or reinstallation of Windows. They cannot be removed by an antivirus as the rootkit directly interacts with the kernel upon boot, long before any antivirus system drivers are being loaded. The rootkit does not patch actual files or drivers, but rather does this dynamically upon booting Windows. A complete format of your system would not get rid of the MBR as it is hosted in the first sectors of your hard drive and therefore survive a format of a single partition.

More information can be found here: http://www2.gmer.net/mbr/.

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